Coherent field transients below 15 THz by difference frequency generation in 4H-SiC

Abstract

The frequency range between 5 THz and 15 THz is a spectral region of particular interest since it includes various fundamental excitations in condensed-matter systems such as phonons in solids, molecular vibrations and low-energy collective modes of correlated materials. However, the generation and coherent detection of highly stable THz transients for ultrafast experiments in this spectral interval is hindered by the fact that most second-order nonlinear materials display a forbidden region due to the presence of the Reststrahlen band and also a lack of birefringence for proper phase matching. In this work, we present 4H silicon carbide (4H-SiC) as a new nonlinear crystal for difference frequency generation (DFG) in this elusive spectral regime. SiC features a large variety of advantageous properties including a Reststrahlen band at relatively high frequencies between 20 THz and 50 THz [1] and thus complete transparency from 17 THz to below 1 THz as well as excellent transmission in the visible range without two-photon absorption of near-infrared pump light. Furthermore, high-quality crystals of outstanding thermal and mechanical robustness are available. Large second-order nonlinearity [2, 3] plus strong uniaxial birefringence allow for phase matching and high conversion efficiency in thick crystals.